The fabrication of optical fibers typically comprises two distinct steps. First, a preform is made. Then, in an entirely separate process, the fiber is drawn from the preform. This two-step procedure has a number of limitations. For example, preforms, from which step index and graded index optical fibers are drawn, can be manufactured in any one of a variety of ways. One such way is the so-called "modified chemical vapor deposition" (MCVD) process in which a plurality of layers of suitably doped silicate glasses are deposited on the inside of a fused silica tube. After the requisite number of layers (i.e., typically about 50) have been deposited, the tube is collapsed into a solid rod which constitutes the preform from which a fiber can be drawn. For a more detailed discussion of the MCVD process, see the article by J. B. Mac Chesney, P. B. O'Connor and H. M. Presby entitled "A New Technique for the Preparation of Low-Loss and Graded-Index Optical Fibers," published in the Proceedings of the IEEE, September 1974, pp. 1280-1281. Also see, "Preparation and Structural Characteristics of High Silica, Graded Index Fibers" by P. B. O'Connor, J. B. Mac Chesney, H. M. Presby and L. G. Cohen, published in The American Ceramic Society Bulletin, Vol. 55, No. 5, May 1976, pp. 513-519.
In an alternative method, described in U.S. Pat. No. 3,966,446, the preform is fabricated by the axial deposition of particulate glass precursor material, the so-called "vapor-phase axial-deposition" (VAD) method. See also the article entitled "Low OH Content Optical Fibre Fabrication By Vapor-Phase Axial-Deposition Method," published in the Aug. 17, 1978 issue of Electronics Letters, pp. 534-535.
As can be seen, the preform fabrication process is a lengthy and costly one. While the ultimate cost of mass produced optical fibers can only be estimated at this time, it is fairly well established that the cost of the preform is approximately 50 to 70 percent of the total cost of the fiber.
In addition, preform fabrication is a demanding process inasmuch as the ultimate characteristics of the fiber are frozen into the preform. While various techniques have been devised for monitoring a fiber as it is drawn from the preform, these monitoring techniques are limited to controlling the size and shape of the fiber, but not its index profile which, as noted above, is defined by the preform. Accordingly, means have been sought for inspecting the preform before the fiber is drawn, thus providing the fiber fabricator with at least an opportunity to discard those preforms that are clearly defective. While this represents a saving insofar as it eliminates the costs associated with drawing, coating and testing fibers drawn from faulty preforms, it is clear that an even greater saving could be realized if the preform fabricating step itself was substantially simplified and controllable.